Studies to find out new functions of a substance through application of ultra-high pressure thereto have been widely carried out around the world.
For example, an organic superconductor (TMTSF)2PF6 was found on the basis of studies on the pressure-dependency of metal-nonmetal transition, and an 8K superconductor β-(BEDT-TTF)2I3 was found through studies on the pressure-dependency of characteristics of the substance (for example, see NPLs 1 and 2).
Novel superconductors of these substances expressed the characteristic of superconductivity under 1 GPa or less, but a superconductor of β′-(BEDT-TTF)2ICl2 was found under a high pressure of 8 GPa, and showed a transition temperature of 14 K that is the highest among organic conductors (for example, see NPL 3).
Development of new substances has been carried out through investigation and resolution of change in physical properties by pressure change with respect to not only such organic superconductors but also solid substances such as oxide conductors and the like.
In general, as a means for applying an ultrahigh pressure to a substance the substance must be pressurized gently and uniformly, and therefore in many cases, the object substance is given a pressure via a pressure medium oil that is a liquid pressure medium.
Regarding the characteristics that are required for the pressure medium oil for measurement under high pressure as mentioned above, the first is that the pressure medium oil does not solidify throughout a broad pressure range and can maintain a liquid state. In other words, when a pressure medium oil solidifies during pressure application, it provides monoaxial compression and fails in uniform compression.
Next, in the case of electric conductivity measurement under pressure, a conductive paste is often used as an electrode, and a pressure medium oil is required to have a characteristic of not dissolving the conductive paste.
Further, when cooled down to be at a temperature not higher than room temperature, a pressure medium oil solidifies at such a low temperature even though it is liquid during pressurization. When a great pressure change occurs during solidification, a brittle sample may be broken. Accordingly, it is further desired that a pressure medium oil has a low pour point as another characteristic.
As still other necessary characteristics thereof, it is also desired that a pressure medium oil has a small compressibility in order that a wall does not contact with a sample during compression.
In the case where a sample of a porous substance such as zeolite is analyzed for pressure effect thereto, it is desired that a pressure medium oil does not come in the space of the pores of the porous substance. It has been verified that helium, and a mixed liquid of methanol and ethanol has extremely good hydrostatic pressure performance, but the molecular size of these substances is smaller than the pore size of a porous substance, and it is often difficult to investigate the pressure characteristics of a porous substance. Accordingly, it is desired that the molecular size of a pressure medium oil is large than the pore size of a porous substance.
Various developments have been made regarding pressure medium oils capable of satisfying these required characteristics.
For example, PTL 1 discloses an invention relating to a pressure medium oil of a hydrocarbon compound and/or an ether compound whose kinematic viscosity at 40° C., viscosity index, density, and pour point each fall within a specific range. PTL 1 says that the solidification pressure at room temperature of the pressure medium oil reached 2.7 GPa.
PTL 2 discloses an invention relating to a pressure medium oil of a silicon-containing organic compound whose kinematic viscosity at 40° C., viscosity index, and pour point each fall within a specific range. PTL 2 says that the solidification pressure at room temperature of the pressure medium oil reached 3.7 GPa.